Differences in hydrocarbon generation kinetics between shale and tar-rich coal and temperature conditions for in-situ co-production

The area of shale exhibiting middle- to low-maturity constitutes over 90% of the total shale area within the seventh Member of the Yanchang Formation (Chang 7 Member) in the Ordos Basin. Additionally, the tar-rich coal resources of the Lower-Jurassic Yan’an Formation are estimated to be approximately 141.4 billion tons. In-situ conversion and co-production technology is an important way to realize synergistic exploitation of the Chang 7 Member shale and Yan’an Formation tar-rich coal. Previous studies have been conducted on the hydrocarbon generation kinetics of shale and coal, but the differences in geochemical and hydrocarbon generation kinetic characteristics between shale and tar-rich coal are still unclear. This restricts the accurate evaluation of in-situ conversion conditions and resource potential of tar-rich coal. Based on the comparative analysis of geochemical characteristics of the Chang 7 Member shale and Yan’an Formation tar-rich coal, this study selected four shale samples with similar maturity and different organic matter abundance, as well as four tar-rich coal samples with similar maturity and different tar yields, to compare and analyze the pyrolysis hydrocarbon generation conversion rate and activation energy distributions of shale and tar-rich coal. The kinetic parameters obtained from laboratory conditions are extrapolated to the in-situ conversion conditions, the feasibility and in-situ conversion temperature conditions of shale oil and tar co-production are analyzed. The results show that both the Chang 7 Member shale and Yan’an Formation tar-rich coal have the conditions for oil generation, with the shale demonstrating superior potential compared to the tar-rich coal. Under conditions of similar maturation, the activation energy of shale increases with organic matter abundance, whereas the activation energy of tar-rich coal decreases with higher tar yield. Furthermore, the distribution of activation energy for the Yan’an Formation tar-rich coal is more dispersed than that of the Chang 7 Member shale. In terms of in-situ conversion, the temperature corresponding to the main hydrocarbon generation stage of shale with similar maturity increases with the increase of organic matter abundance, while the temperature for tar-rich coal decreases with higher tar yield. The temperature range (226-443 ℃) corresponding to the main hydrocarbon generation stage of the Chang 7 Member shale (TOC=5%-38%，Ro=0.68%-0.71%) covers the temperature range (282-435 ℃) corresponding to the main hydrocarbon generation stage of the Yan’an Formation tar-rich coal (Tar,d=7%-15.3%，Ro=0.67%-0.72%). Therefore, the in-situ pyrolysis co-production of the Chang 7 Member shale oil and Yan’an Formation tar can be achieved when the pyrolysis temperature reaches the temperature corresponding to the main hydrocarbon generation stage of the shale in the Ordos Basin.